The Install Tool provides a convenient user interface for changing
the global configuration. The “All Configuration” view lists all
available options and makes them available for editing.

When changes are saved, the LocalConfiguration.php file
is updated.

Note how the configuration options description is taken from the
inline comments in the typo3/sysext/core/Configuration/DefaultConfiguration.php
file. Check out the code again:

returnarray('GFX'=>array(// Configuration of the image processing features in TYPO3. 'IM' and 'GD' are short for ImageMagick and GD library respectively.'image_processing'=>true,// Boolean: Enables image processing features. Disabling this means NO image processing with either GD or IM!'thumbnails'=>true,// Boolean: Enables the use of thumbnails in the backend interface.'thumbnails_png'=>0,// Bits. Bit0: If set, thumbnails from non-jpegs will be 'png', otherwise 'gif' (0=gif/1=png). Bit1: Even JPG's will be converted to png or gif (2=gif/3=png)'gif_compress'=>true,// Boolean: Enables the use of the \TYPO3\CMS\Core\Utility\GeneralUtility::gifCompress() workaround function for compressing giffiles made with GD or IM, which probably use only RLE or no compression at all....),...);